1. Field of the Invention
This invention relates to photosynthetic hydrogen production and specifically to a biophotolysis process, which can be cycled, for the temporal separation of oxygen evolution and hydrogen production in algae.
2. Description of the Related Art
New clean energy sources that are free of environmental pollution have been sought worldwide as a substitute for fossil fuels. Among the potential sources, the photobiological production of hydrogen by procaryotic or eucaryotic organisms is a desirable way of generating a renewable hydrogen fuel from light and water, which are among nature's most plentiful resources.
The ability of green algae, such as Chlamydomonas reinhardtii, to produce hydrogen from water has been recognized for over 55 years. This reaction is catalyzed by the reversible hydrogenase, an enzyme that is induced in the cells after exposure to a short period of anaerobiosis. However, the activity is rapidly lost, as soon as the light is turned on, because of immediate inactivation of the reversible hydrogenase by photosynthetically generated O2.
In the prior art, certain methods have been used to circumvent the inactivation problem. U.S. Pat. No. 4,532,210 discloses the biological production of hydrogen in a algal culture using an alternating light and dark cycle. The process comprises alternating a step for cultivating the alga in water under aerobic conditions in the presence of light to accumulate photosynthetic products (starch) in the alga and a step for cultivating the alga in water under microaerobic conditions in the dark to decompose the material accumulated by photosynthesis to evolve hydrogen. This method uses a nitrogen gas purge technique to remove oxygen, carried over from the light cycle, from the culture.
U.S. Pat. No. 4,442,211 discloses that the efficiency of a process for producing hydrogen, by subjecting algae in an aqueous phase to light irradiation, is increased by culturing algae which has been bleached during a first period of irradiation in a culture medium in an aerobic atmosphere until it has regained color and then subjecting these algae to a second period of irradiation wherein hydrogen is produced at an enhanced rate. A reaction cell is used in light irradiating the culture in an environment that is substantially free of CO2 and atmospheric O2. This environment is maintained by passing an inert gas (e.g. helium) through the cell to remove all hydrogen and oxygen generated by the splitting of water molecules in the aqueous medium. Although continuous purging of H2-producing cultures with inert gases has allowed for the sustained production of H2, such purging is expensive and impractical for large-scale mass cultures of algae.
The use of exogenous reductants, such as sodium dithionite, as well as the addition of herbicides to inhibit photosynthetic O2 evolution, has also been used, but these methods are either impractical or create an irreversible condition that may lead to cell death.
An alternative approach to photoproduce hydrogen is based on the concept of indirect biophotolysis in which metabolite accumulation by photosynthesis serves as a substrate for subsequent hydrogen production. In this approach, the two reactions, photosynthesis and H2 production, are spatially and/or temporally, separated from each other. See e.g., Benemann, J. R. Hydrogen Biotechnology: Progress and Prospects. Nature Biotechnol. 14: 1101-1103 (1996).
In view of the foregoing a need exists for a sustainable process of photosynthetic hydrogen production in an algal culture. Unlike the foregoing methods, which rely on a mechanical means or chemical manipulations to the cells, the sustainable process would desirably overcome the hydrogenase oxygen-sensitivity problem through a low cost physiological response.